Recently, optical recording media have come to be in great demand. An optical recording medium generally comprises a transparent substrate and a recording layer formed on one side of the substrate which is capable of being changed by optical energy. The medium is used for recording and reproduction of information by irradiating it with a laser beam at its substrate side. Not only materials of the recording layer but also protective films for preventing the deterioration of the recording layer by water or oxygen are being developed, as disclosed in Japanese Patent Laid-Open No. 68,850/1984.
The protective layer typically is made of a material having low moisture permeability. As inorganic materials, oxide films of SiO.sub.2, MgO and Al.sub.2 O.sub.3 are used. As organic material, polypropylene, ethylene-vinyl acetate copolymer, polyvinylidene chloride, etc. are used.
Concerning the optical recording medium having the above structure, its water-proof means will now be described.
In order to prevent introduction of water through the substrate, oxides or fluorides are provided on the substrate surface. These inorganic materials usually have low moisture transmittance, and therefore they can improve corrosion resistance of the recording layer when the optical recording medium is held in a high relative humidity atmosphere. Further, for preventing corrosion of the recording layer, organic polymers having comparatively low moisture permeability, e.g., polyvinylidene chloride, are used in lieu of the inorganic material. Furthermore, epoxy resins or like crosslinking resins can be used as organic polymer-based protective films, as disclosed in Japanese Patent Laid-Open 70,894/1982.
However, the protective layer as noted above has the following problems.
(1) Inorganic materials such as oxides and fluorides provide weak bonding forces with respect to the transparent substrate, and are prone to separation or generation of cracks when left under high temperature, high relative humidity conditions.
(2) It is difficult to form a dense thin film of an oxide or a fluoride.
(3) With organic polymers, although the moisture permeability is low, there is a limitation on the water content barrier character. Therefore, when the polymers are left under high temperature, high relative humidity conditions for long time, the water-proof effect is lost.
(4) The crosslinking resins have many pores and have high moisture permeability. Therefore, the recording layer is readily deteriorated by water when the optical recording medium is maintained for a long time under high temperature, high relative humidity conditions.
For the above reasons, when the optical recording medium is preserved under high temperature, high relative humidity conditions, it is subject to corroding and deterioration. Therefore, requirements for the water- and corrosion-proof optical recording medium have not been met.